Process for selective removal of H2 S from mixtures containing H22 using di-severely sterically hindered secondary aminoethers

ABSTRACT

The selective removal of H 2  S gas from a normally gaseous mixture containing H 2  S and CO 2  is accomplished by contacting the gaseous mixture with an absorbent solution comprising a di-secondary aminoether wherein each amino group has a severely sterically hindered secondary amino moiety whereby H 2  S is selectively absorbed from the mixture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for selective removal of H₂ S fromgaseous mixtures containing H₂ S and CO₂ wherein a class of di-severelysterically hindered secondary aminoethers are employed.

2. Description of Related Patents

It is well known in the art to treat gases and liquids, such as mixturescontaining acidic gases including CO₂, H₂ S, CS₂, HCN, COS and oxygenand sulfur derivatives of C₁ to C₄ hydrocarbons with amine solutions toremove these acidic gases. The amine usually contacts the acidic gasesand the liquids as an aqueous solution containing the amine in anabsorber tower with the aqueous amine solution contacting the acidicfluid countercurrently.

The treatment of acid gas mixtures containing, inter alia, CO₂ and H₂ Swith amine solutions typically results in the simultaneous removal ofsubstantial amounts of both the CO₂ and H₂ S. For example, in one suchprocess generally referred to as the "aqueous amine process," relativelyconcentrated amine solutions are employed. A recent improvement on thisprocess involves the use of sterically hindered amines as described inU.S. Pat. No. 4,112,052, to obtain nearly complete removal of acid gasessuch as CO₂ and H₂ S. This type of process may be used where the partialpressures of the CO₂ and related gases are low. Another process oftenused for specialized applications where the partial pressure of CO₂ isextremely high and/or where many acid gases are present, e.g., H₂ S,COS, CH₃ SH and CS₂ involves the use of an amine in combination with aphysical absorbent, generally referred to as the "nonaqueous solventprocess." An improvement on this process involves the use of stericallyhindered amines and organic solvents as the physical absorbent such asdescribed in U.S. Pat. No. 4,112,051.

It is often desirable, however, to treat acid gas mixtures containingboth CO₂ and H₂ S so as to remove the H₂ S selectively from the mixture,thereby minimizing removal of the CO₂. Selective removal of H₂ S resultsin a relatively high H₂ S/CO₂ ratio in the separated acid gas whichsimplifies the conversion of H₂ S to elemental sulfur using the Clausprocess.

The typical reactions of aqueous secondary and tertiary amines with CO₂and H₂ S can be represented as follows:

    H.sub.2 S+R.sub.3 N⃡R.sub.3 NH.sup.+ +SH.sup.- ( 1)

    H.sub.2 S+R.sub.2 NH⃡R.sub.2 NH.sub.2.sup.+ +SH.sup.-( 2)

    CO.sub.2 +R.sub.3 N+H.sub.2 O⃡R.sub.3 NH.sup.+ +HCO.sub.3.sup.31                                         ( 3)

    CO.sub.2 2R.sub.2 NH⃡R.sub.2 NH.sub.2.sup.30 +R.sub.2 NCOO.sup.31                                               ( 4)

wherein R is an organic radical which may be the same or different andmay be substituted with a hydroxy group. The above reactions arereversible, and the partial pressures of both CO₂ and H₂ S are thusimportant in determining the degree to which the above reactions occur.

While selective H₂ S removal is applicable to a number of gas treatingoperations including treatment of hydrocarbon gases from shalepyrolysis, refinery gas and natural gas having a low H₂ S/CO₂ ratio, itis particularly desirable in the treatment of gases wherein the partialpressure of H₂ S is relatively low compared to that of CO₂ because thecapacity of an amine to absorb H₂ S from the latter type gases is verylow. Examples of gases with relatively low partial pressures of H₂ Sinclude synthetic gases made by coal gasification, sulfur plant tail gasand low-Joule fuel gases encountered in refineries where heavy residualoil is being thermally converted to lower molecular weight liquids andgases.

Although it is known that solutions of primary and secondary amines suchas monoethanolamine (MEA), diethanolamine (DEA), dipropanolamine (DPA),and hydroxyethoxyethylamine (DGA) absorb both H₂ S and CO₂ gas, theyhave not proven especially satisfactory for preferential absorption ofH₂ S to the exclusion of CO₂ because the amines undergo a facilereaction with CO₂ to form carbamates.

Diisopropanolamine (DIPA) is relatively unique among secondaryaminoalcohols in that it has been used industrially, alone or with aphysical solvent such as sulfolane, for selective removal of H₂ S fromgases containing H₂ S and CO₂, but contact times must be kept relativelyshort to take advantage of the faster reaction of H₂ S with the aminecompared to the rate of CO₂ reaction as shown in Equations 2 and 4hereinabove.

In 1950, Frazier and Kohl, Ind. and Eng. Chem., 42, 2288 (1950) showedthat the tertiary amine, methyldiethanolamine (MDEA), has a high degreeof selectivity toward H₂ S absorption over CO₂. This greater selectivitywas attributed to the relatively slow chemical reaction of CO₂ withtertiary amines as compared to the rapid chemical reaction of H₂ S. Thecommercial usefulness of MDEA, however, is limited because of itsrestricted capacity for H₂ S loading and its limited ability to reducethe H₂ S content to the level at low pressures which is necessary fortreating, for example, synthetic gases made by coal gasification.

Recently, U.K. Pat. No. 2,017,524A to Shell disclosed that aqueoussolutions of dialkylmonoalkanolamines, and particularlydiethylmonoethanolamine (DEAE), have higher selectivity and capacity forH₂ S removal at higher loading levels than MDEA solutions. Nevertheless,even DEAE is not very effective for the low H₂ S loading frequentlyencountered in the industry. Also, DEAE has a boiling point of only 161°C., therefore it is characterized as being a low-boiling, relativelyhighly volatile amino alcohol. Such high volatilities under most gasscrubbing conditions result in large material losses with consequentlosses in economic advantages.

SUMMARY OF THE INVENTION

It has now been discovered that absorbent solutions of a certain classof amino compounds defined as di-severely sterically hindered secondaryaminoethers have a high selectivity for H₂ S compared to CO₂. Theseamino compounds surprisingly maintain their high selectivity at high H₂S and CO₂ loadings.

The di-severely sterically hindered secondary aminoethers herein aremono- or polyether compounds wherein each amino group has a severelysterically hindered secondary amino moiety.

By the term "severely sterically hindered" it is meant that the nitrogenatom of the amine is attached to one or more bulky carbon groupings.Typically, these secondary di-aminoethers will have a degree of sterichindrance such that the cumulative -E_(s) value (Taft's steric hindranceconstant) is greater than about 1.75 as calculated from the values givenfor primary amines in Table V from the article by D. F. DeTar, Journalof Organic Chemistry, 45, 5174 (1980), the entire disclosure of which isincorporated herein by reference.

The di-severely hindered secondary aminoethers of this invention arespecifically referred to herein as di-(alkylaminoalkyl)ethersrepresented by the following general formula: ##STR1## wherein R₁ and R₈are each independently selected from the group consisting of alkylhaving 1 to 8 carbon atoms and hydroxyalkyl radicals having 2 to 8carbon atoms, R₂, R₃, R₄, R₅, R₆, and R₇ are each independentlyselectedfrom the group consisting of hydrogen, C₁ -C₄ alkyl and hydroxyalkylradicals, with the proviso that R₂, R₃, R₆ and R₇ bonded to the carbonatoms directly bonded to the nitrogen atoms are C₁ -C₄ alkyl orhydroxyalkyl radicals when R₁ and R₈ are primary alkyl or primaryhydroxyalkyl radicals, and when R₁ and R₈ are secondary alkyl orsecondary hydroxyalkyl radicals, at least one of R₂ or R₃ and R₆ or R₇bonded to the carbon atom directly bonded to the nitrogen atoms arealkyl or hydroxy radicals, and when R₂, R₃, R₆, and R₇ are hydrogen R₁and R₈ are tertiary alkyl or tertiary hydroxyalkyl radicals having 4 to8 carbon atoms, m, n and p are positive integers ranging from 2 to 4,and o is either zero or a positive integer ranging from 1 to 10.Preferably, R₁ and R₈ are selected from the group consisting of tertiarybutyl, tertiary amyl, and cycloalkyl radicals, e.g., cyclopentyl andcyclohexyl and isopropyl radicals. Most preferably, R₁ and R₈ are bothtertiary butyl radicals and R₂, R₃, R₄, R₅, R₆ and R₇ are hydrogen. WhenR₁ and R₈ are secondary alkyl radicals, e.g., isopropyl, it is preferredthat each of R₂ and R₆ is a methyl radical. The symbols m, n and p arepreferably 2, whereas the symbol o is preferably zero or 1 or 2.

Representative di-secondary aminoethers include, for example,bis-(tertiarybutylaminoethyl)ether,1,2-bis-(tertiarybutylaminoethoxy)ethane,1,2-bis-(tertiarybutylaminoethoxyethoxy)ethane,bis[2-(isopropylamino)propyl]ether,1,2-[2-(isopropylamino)propoxy]ethane and the like.

In particular, the present invention relates to a process for theselective absorption of H₂ S from a normally gaseous mixture containingH₂ S and CO₂ comprising:

(a) contacting said normally gaseous mixture with an absorbentamino-compound-containing solution comprising a di-secondary aminoetherwherein each amino group has a severely sterically hindered secondaryamino moiety under conditions such that H₂ S is selectively absorbedfrom said mixture;

(b) regenerating, at least partially, said absorbent solution containingH₂ S; and

(c) recycling the regenerated solution for the selective absorption ofH₂ S by contacting as in step (a).

Preferably, the regeneration step is carried out by heating andstripping and more preferably heating and stripping with steam.

One of the preferred amino compounds is1,2-bis-(tertiarybutylaminoethoxy)ethane.

The amino compounds herein are further characterized by their lowvolatility and high solubility in water at selective H₂ S removalconditions, and most of the compounds are also generally soluble inpolar organic solvent systems which may or may not contain water. Theterm "absorbent solution" as used herein includes but is not limited tosolutions wherein the amino compound is dissolved in a solvent selectedfrom water or a physical absorbent or mixtures thereof. Solvents whichare physical absorbents (as opposed to the amino compounds which arechemical absorbents) are described, for example, in U.S. Pat. No.4,112,051, the entire disclosure of which is incorporated herein byreference, and include, e.g., aliphatic acid amides, N-alkylatedpyrrolidones, sulfones, sulfoxides, glycols and the mono- and diethersthereof. The preferred physical absorbents herein are sulfones, and mostparticularly, sulfolane.

The absorbent solution ordinarily has a concentration of amino compoundof about 0.1 to 6 moles per liter of the total solution, and preferably1 to 4 moles per liter, depending primarily on the specific aminocompound employed and the solvent system utilized. If the solvent systemis a mixture of water and a physical absorbent, the typical effectiveamount of the physical absorbent employed may vary from 0.1 to 5 molesper liter of total solution, and preferably from 0.5 to 3 moles perliter, depending mainly on the type of amino compound being utilized.The dependence of the concentration of amino compound on the particularcompound employed is significant because increasing the concentration ofamino compound may reduce the basicity of the absorbent solution,thereby adversely affecting its selectivity for H₂ S removal,particularly if the amino compound has a specific aqueous solubilitylimit which will determine maximum concentration levels within the rangegiven above. It is important, therefore, that the proper concentrationlevel appropriate for each particular amino compound be maintained toinsure satisfactory results.

The solution of this invention may include a variety of additivestypically employed in selective gas removal processes, e.g., antifoamingagents, antioxidants, corrosion inhibitors, and the like. The amount ofthese additives will typically be in the range that they are effective,i.e., an effective amount.

Also, the amino compounds described herein may be admixed with otheramino compounds as a blend, preferably with methyldiethanolamine. Theratio of the respective amino compounds may vary widely, for example,from 1 to 99 weight percent of the amino compounds described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic flow sheet illustrating anabsorption-regeneration unit for selective removal of H₂ S from gaseousstreams containing H₂ S and CO₂.

FIG. 2 is a diagrammatic flow sheet illustrating an experimental spargedabsorber unit for use in rapid determination of the selectivity of theamino compound for selective removal of H₂ S from gaseous streamscontaining H₂ S and CO₂.

FIG. 3 graphically illustrates the selectivity for H₂ S plotted againstthe H₂ S and CO₂ loading for 1,2-(bis-tertiarybutylaminoethoxy)ethane(BIS-TB) as compared to tertiarybutylaminoethoxyethanol (TBEE),1,2-bis-(isopropylaminoethoxy)ethane (BIS-IP), and methyldiethanolamine(MDEA).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The di-secondary aminoethers used in the practice of this invention arepreferably represented by the following general formula: ##STR2##wherein R₁ and R₈ are each independently selected from the groupconsisting of secondary alkyl or secondary hydroxyalkyl radicals having3 to 8 carbon atoms, or tertiary alkyl or tertiary hydroxyalkyl radicalshaving 4 to 8 carbon atoms, R₂ and R₆ are each independently selectedfrom the group consisting of hydrogen and C₁ -C₄ alkyl radicals, withthe proviso that when R₁ and R₈ are secondary alkyl radicals, R₂ and R₆are C₁ -C₄ alkyl radicals, and o is either zero or a positive integerranging from 1 to 4.

Typical compounds of the invention defined by the general formula aboveinclude: ##STR3##

These di-secondary aminoethers may be prepared by reacting theappropriate primary amine, e.g., R₁ --NH₂ and R₈ --NH₂ with adi-(haloalkoxy)alkane or bis-sulfonate ester of a glycolic ether underconditions such that the haloacid or sulfonic acid is eliminated. Forexample, 1,2-bis-(t-butylaminoethoxy)ethane may be prepared by reactingt-butylamine with 1,2-(bis-chloroethoxy)ethane in a solvent such asethylene glycol monomethyl ether under reflux conditions. The productcan be simply recovered by treating the hot solution with an excessamount of an alkaline hydroxide followed by filtration and distillationof the product.

The amino compounds used in the process of the present invention have apK_(a) value at 20° C. greater than 8.6, preferably greater than about9.5 and more preferably the pK_(a) value of the amino compound willrange between about 9.5 and about 10.6. If the pK_(a) is less than 8.6the reaction with H₂ S is decreased, whereas if the pK_(a) of the aminocompound is much greater than about 10.6 an excessive amount of steam isrequired to regenerate the solution. Also, to insure operationalefficiency with minimal losses of the amino compound, the amino compoundshould have a relatively low volatility. For example, the boiling pointof the amine (at 760 mm) is typically greater than about 180° C.,preferably greater than 200° C., and more preferably greater than 225°C.

Three characteristics which are of ultimate importance in determiningthe effectiveness of the amino compounds herein for H₂ S removal are"selectivity", "loading" and "capacity". The term "selectivity" as usedthroughout the specification is defined as the following mole ratiofraction: ##EQU1## The higher this fraction, the greater the selectivityof the absorbent solution for the H₂ S in the gas mixture.

By the term "loading" is meant the concentration of the H₂ S and CO₂gases physically dissolved and chemically combined in the absorbentsolution as expressed in moles of gas per moles of the amine. The bestamino compounds are those which exhibit good selectivity up to arelatively high loading level. The amino compounds used in the practiceof the present invention typically have a "selectivity" of notsubstantially less than 10 at a "loading" of 0.1 moles, preferably, a"selectivity" of not substantially less than 10 at a loading of 0.2 ormore moles of H₂ S and CO₂ per moles of the amino compound.

"Capacity" is defined as the moles of H₂ S loaded in the absorbentsolution at absorption conditions less the moles of H₂ S loaded in theabsorbent solution at desorption conditions. High capacity enables oneto reduce the amount of amine solution to be circulated and use lessheat or steam during regeneration.

The acid gas mixture herein necessarily includes H₂ S, and mayoptionally include other gases such as CO₂, N₂, CH₄, H₂, CO, H₂ O, COS,HCN, C₂ H₄, NH₃, and the like. Often such gas mixtures are found incombustion gases, refinery gases, town gas, natural gas, syn gas, watergas, propane, propylene, etc. The absorbent solution herein isparticularly effective when the gaseous mixture is a low-Joule gas,obtained, for example, from shale oil retort gas, coal or gasificationof heavy oil (POX), thermal conversion of heavy residual oil to lowermolecular weight liquids and gases, or in sulfur plant tail gas clean-upoperations.

The absorption step of this invention generally involves contacting thenormally gaseous stream with the absorbent solution in any suitablecontacting vessel. In such processes, the normally gaseous mixturecontaining H₂ S and CO₂ from which the H₂ S is to be selectively removedmay be brought into intimate contact with the absorbent solution usingconventional means, such as a tower or vessel packed with, for example,rings or with sieve plates, or a bubble reactor.

In a typical mode of practicing the invention, the absorption step isconducted by feeding the normally gaseous mixture into the lower portionof the absorption tower while fresh absorbent solution is fed into theupper region of the tower. The gaseous mixture, freed largely from theH₂ S, emerges from the upper portion of the tower, and the loadedabsorbent solution, which contains the selectively absorbed H₂ S, leavesthe tower near or at its bottom. Preferably, the inlet temperature ofthe absorbent solution during the absorption step is in the range offrom about 20° to about 100° C., and more preferably from 40° to about60° C. Pressures may vary widely; acceptable pressures are between 5 and2000 psia, preferably 20 to 1500 psia, and most preferably 25 to 1000psia in the absorber. The contacting takes place under conditions suchthat the H₂ S is selectively absorbed by the solution. The absorptionconditions and apparatus are designed so as to minimize the residencetime of the liquid in the absorber to reduce CO₂ pickup while at thesame time maintaining sufficient residence time of gas mixture withliquid to absorb a maximum amount of the H₂ S gas. The amount of liquidrequired to be circulated to obtain a given degree of H₂ S removal willdepend on the chemical structure and basicity of the amino compound andon the partial pressure of H₂ S in the feed gas. Gas mixtures with lowpartial pressures such as those encountered in thermal conversionprocesses will require more liquid under the same absorption conditionsthan gases with higher partial pressures such as shale oil retort gases.

A typical procedure for the selective H₂ S removal phase of the processcomprises selectively absorbing H₂ S via countercurrent contact of thegaseous mixture containing H₂ S and CO₂ with the aqueous solution of theamino compound in a column containing a plurality of trays at a lowtemperature, e.g., below 45° C., and at a gas velocity of at least about0.3 ft/sec (based on "active" or aerated tray surface), depending on theoperating pressure of the gas, said tray column having fewer than 20contacting trays, with, e.g., 4-16 trays being typically employed.

After contacting the normally gaseous mixture with the absorbentsolution, which becomes saturated or partially saturated with H₂ S, thesolution may be at least partially regenerated so that it may berecycled back to the absorber. As with absorption, the regeneration maytake place in a single liquid phase. Regeneration or desorption of theacid gases from the absorbent solution may be accomplished byconventional means such as pressure reduction of the solution orincrease of temperature to a point at which the absorbed H₂ S flashesoff, or by passing the solution into a vessel of similar construction tothat used in the absorption step, at the upper portion of the vessel,and passing an inert gas such as air or nitrogen or preferably steamupwardly through the vessel. The temperature of the solution during theregeneration step should be in the range from about 50° to about 170°C., and preferably from about 80° to 120° C., and the pressure of thesolution on regeneration should range from about 0.5 to about 100 psia,preferably 1 to about 50 psia. The absorbent solution, after beingcleansed of at least a portion of the H₂ S gas, may be recycled back tothe absorbing vessel. Makeup absorbent may be added as needed.

In the preferred regeneration technique, the H₂ S-rich solution is sentto the regenerator wherein the absorbed components are stripped by thesteam which is generated by re-boiling the solution. Pressure in theflash drum and stripper is usually 1 to about 50 psia, preferably 15 toabout 30 psia, and the temperature is typically in the range from about50° to 170° C., preferably about 80° to 120° C. Stripper and flashtemperatures will, of course, depend on stripper pressure, thus at about15 to 30 psia stripper pressures, the temperature will be about 80° toabout 120° C. during desorption. Heating of the solution to beregenerated may very suitably be effected by means of indirect heatingwith low-pressure steam. It is also possible, however, to use directinjection of steam.

In one embodiment for practicing the entire process herein, asillustrated in FIG. 1, the gas mixture to be purified is introducedthrough line 1 into the lower portion of a gas-liquid countercurrentcontacting column 2, said contacting column having a lower section 3 andan upper section 4. The upper and lower sections may be segregated byone or a plurality of packed beds as desired. The absorbent solution asdescribed above is introduced into the upper portion of the columnthrough a pipe 5. The solution flowing to the bottom of the columnencounters the gas flowing countercurrently and dissolves the H₂ Spreferentially. The gas freed from most of the H₂ S exits through a pipe6 for final use. The solution, containing mainly H₂ S and some CO₂,flows toward the bottom portion of the column, from which it isdischarged through pipe 7. The solution is then pumped via optional pump8 through an optional heat exchanger and cooler 9 disposed in pipe 7,which allows the hot solution from the regenerator 12 to exchange heatwith the cooler solution from the absorber column 2 for energyconservation. The solution is entered pipe 7 to a flash drum 10 equippedwith a line (not shown) which vents to line 13 and then introduced bypipe 11, into the upper portion of the regenerator 12, which is equippedwith several plates and effects the desorption of the H₂ S and CO₂ gasescarried along in the solution. This acid gas mixture is passed through apipe 13 into a condenser 14 wherein cooling and condensation of waterand amine solution from the gas occur. The gas then enters a separator15 where further condensation is effected. The condensed solution isreturned through pipe 16 to the upper portion of the regenerator 12. Thegas remaining from the condensation, which contains H₂ S and some CO₂,is removed through pipe 17 for final disposal (e.g., to a vent orincinerator or an apparatus which converts the H₂ S to sulfur, such as aClaus unit or a Stretford conversion unit (not shown)).

The solution is liberated from most of the gas which it contains whileflowing downward through the regenerator 12 and exits through pipe 18 atthe bottom of the regenerator for transfer to a reboiler 19. Reboiler19, equipped with an external source of heat (e.g., steam injectedthrough pipe 20 and the condensate exits through a second pipe (notshown)), vaporizes a portion of this solution (mainly water) to drivefurther H₂ S therefrom. The H₂ S and steam driven off are returned viapipe 21 to the lower section of the regenerator 12 and exited throughpipe 13 for entry into the condensation stages of gas treatment. Thesolution remaining in the reboiler 19 is drawn through pipe 22, cooledin heat exchanger 9, and introduced via the action of pump 23 (optionalif pressure is sufficiently high) through pipe 5 into the absorbercolumn 2.

The amino compounds herein are found to be superior to those used in thepast, particularly to MDEA and DEAE, in terms of both selectivity andcapacity for maintaining selectivity over a broad loading range.Typically, a gaseous stream to be treated having a 1:10 mole ratio of H₂S:CO₂ from an apparatus for thermal conversion of heavy residual oil, ora Lurgi coal gas having a mole ratio of H₂ S:CO₂ of less than 1:10 willyield an acid gas having a mole ratio of H₂ S:CO₂ of about 1:1 aftertreatment by the process of the present invention. The process hereinmay be used in conjunction with another H₂ S selective removal process;however, it is preferred to carry out the process of this invention byitself, since the amino compounds are extremely effective by themselvesin preferential absorption of H₂ S.

The invention is illustrated further by the following examples, which,however, are not to be taken as limiting in any respect. All parts andpercentages, unless expressly stated to be otherwise, are by weight.

EXAMPLE 1

FIG. 2 illustrates the sparged absorber unit, operated on a semi-batchmode, used to evaluate the selectivity for H₂ S removal of the aminocompounds of the invention herein. A gas mixture comprised of 10% CO₂,1% H₂ S and 89% N₂, expressed in volume percent, respectively was passedfrom a gas cylinder (not shown) through line 30 to a meter 31 measuringthe rate at which the gas is fed to the absorber. For all examples thisrate was 3.6 liters per minute. The gas was then passed through line 32to a gas chromatography column (not shown) continuously monitoring thecomposition of the inlet gas and through lines 33 and 34 to a spargedabsorber unit 35, which is a cylindrical glass tube 45 cm high and 3.1cm in diameter charged with 100 ml of the absorbent amine solution 36.The gas was passed through the solution at a solution temperature of 40°C., and 10-ml samples of the solution were periodically removed from thebottom of the absorber unit through lines 34 and 37 to be analyzed forH₂ S and CO₂ content. The H₂ S content in the liquid sample wasdetermined by titration with silver nitrate. The CO₂ content of theliquid sample was analyzed by acidifying the sample with an aqueoussolution of 10% HCl and measuring the evolved CO₂ by weight gain onNaOH-coated asbestos.

While the solution was being periodically withdrawn from the bottom ofthe absorber unit, the gas mixture was removed from the top thereof vialine 38 to a trap 39 which served to scrub out any H₂ S in the outletgas. The resulting gas could optionally then be passed via lines 40 and41 for final disposal or via line 42 to a gas chromatography column (notshown) for periodic evaluation of the composition of the outlet gas tocheck for system leaks. For purposes of the examples, the H₂ S and CO₂contents of the inlet phase were measured, the H₂ S and CO₂ contents ofthe liquid phase were determined as described above. These data wereused to calculate selectivity values of the amine as defined above,which were plotted as a function of the loading of the absorbentsolution with H₂ S and CO₂, in units of moles acid gas per moles of theamino compound.

In this example an aqueous 1.5 M solution of1,2-bis-(tertiarybutylaminoethoxy)ethane (BIS-TB) (pK_(a) =10.2,b.p.=104.7° C. (0.25 mm)) was employed as the absorbent solution. Theselectivity data were plotted and are shown in FIG. 3.

EXAMPLE 2

The procedure of Example 1 was repeated using as the absorbent solutiona 3 M aqueous solution of tertiarybutylaminoethoxyethanol (TBEE) and theselectivity data were plotted on the graph as shown in FIG. 3.

EXAMPLE 3

The procedure in Example 1 was repeated using as the absorbent solutiona 1.5 M aqueous solution of 1,2-bis-(isopropylaminoethoxy)ethane(BIS-IP) (pK_(a) =10.13, b.p.=85° C. (0.1 mm)) and the selectivity datawere plotted on the graph as shown in FIG. 3.

EXAMPLE 4

The procedure of Example 1 was repeated using as the absorbent solutiona 3 M aqueous solution of methyldiethanolamine (MDEA) and theselectivity data were plotted on the graph as shown in FIG. 3.

The results shown in FIG. 3 indicate that the di-severely stericallyhindered secondary aminoethers of the present invention are far superiorto the di-secondary aminoether that is not severely sterically hindered,the monoamino severely sterically hindered amino alcohol, TBEE, and thetertiary amine, MDEA.

While all examples herein illustrate the superior performance of theamino compounds for selective H₂ S removal using an absorber unit asrepresented by FIG. 2, it will also be possible to achieve effectiveselective H₂ S removal by using the amino compounds in anabsorption-regeneration unit as depicted in FIG. 1.

In summary, this invention is seen to provide a special class of aminocompounds characterized as bis-severely sterically hindered secondaryaminoethers having a high selectivity for H₂ S in preference to CO₂which selectivity is maintained at high H₂ S and CO₂ loading levels.

These amino compounds are capable of reducing the H₂ S in gaseousmixtures to a relatively low level, e.g., less than about 200 ppm andhave a relatively high capacity for H₂ S, e.g., greater than about 0.5mole of H₂ S per mole of amine. The amino compounds are characterized ashaving a "kinetic selectivity" for H₂ S, i.e., a faster reaction ratefor H₂ S than for CO₂ at absorption conditions. In addition they have ahigher capacity for H₂ S at equivalent kinetic selectivity for H₂ S overCO₂. This higher capacity results in the economic advantage of lowersteam requirements during regeneration.

The data in FIG. 3 also show that the amino compounds of the presentinvention have very high capacity for both H₂ S and CO₂ compared tomethyldiethanolamine (MDEA) in addition to high H₂ S selectivities. Itwill be apparent from an inspection of the data in FIG. 3 that if theabsorption process is conducted under conditions such that the aminocompound has a long contact time with the gases to be absorbed, theselectivity for H₂ S decreases, but the overall capacity for both CO₂and H₂ S remains rather high. Therefore, one may, in some instances,wish to carry out a "non-selective" absorption process to take advantageof the large absorption capacity of the amino compounds of theinvention. Accordingly, one may carry out a "non-selective" acid gasremoval absorption process using the amino compounds of the invention.Such "non-selective" processes are particularly useful in scrubbingnatural gases which contain relatively high levels of H₂ S and low tonil levels of CO₂. As such, the amino compounds of the invention mayreplace some or all of monoethanolamine (MEA) or diethanolamine (DEA)commonly used for such scrubbing processes.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures herein before set forth, and as fall within the scope of theinvention.

What is claimed is:
 1. A process for the selective absorption of H₂ S from a normally gaseous mixture containing H₂ S and CO₂ comprising contacting said normally gaseous mixture with an absorbent amino-compound-containing solution comprising a di-secondary aminoether wherein each amino group is severely sterically hindered under conditions whereby H₂ S is selectively absorbed from said mixture.
 2. The process of claim 1 wherein the absorbent solution further comprises water, a physical absorbent, of mixtures thereof.
 3. The process of claim 2 wherein the total concentration of amino compound in the solution is in the range from about 0.1 to about 6 moles per liter.
 4. The process of claim 2 wherein the total concentration of amino compound in the solution is in the range from about 1 to about 4 moles per liter.
 5. The process of claim 1 wherein the absorbent solution is regenerated by heating and stripping.
 6. The process of claim 5 wherein the contacting is conducted at a temperature ranging from about 20° to 100° C. and at a pressure ranging from 5 to 2000 psia.
 7. The process of claim 5 wherein the contacting is conducted at a temperature ranging from about 40° to 60° C. and at a pressure ranging from 20 to 1500 psia.
 8. The process of claim 1 wherein the amino compound is represented by the following general formula: ##STR4## wherein R₁ and R₈ are each independently selected from the group consisting of alkyl having 1 to 8 carbon atoms and hydroxyalkyl radicals having 2 to 8 carbon atoms, R₂, R₃, R₄, R₅, R₆ and R₇ are each independently selected from the group consisting of hydrogen, C₁ -C₄ alkyl and hydroxyalkyl radicals, with the proviso that R₂, R₃, R₆ and R₇ bonded to the carbon atoms directly bonded to the nitrogen atoms are C₁ -C₄ alkyl or hydroxyalkyl radicals when R₁ and R₈ are primary alkyl or primary hydroxyalkyl radicals, and when R₁ and R₈ are secondary alkyl or secondary hydroxyalkyl radicals, at least one of R₂ or R₃ and R₆ or R₇ bonded to the carbon atoms directly bonded to the nitrogen atoms are C₁ -C₄ alkyl or hydroxyalkyl and when R₂, R₃, R₆ and R₇ are hydrogen, R₁ and R₈ are tertiary alkyl or tertiary hydroxyalkyl radicals having 4 to 8 carbon atoms m, n and p are positive integers ranging from 2 to 4 and o is either zero or a positive integer ranging from 1 to
 10. 9. The process of claim 1 wherein the amino compound is bis-(tertiarybutylaminoethyl)ether, 1,2-bis-(tertiarybutylaminoethoxy)ethane, and bis-(2-isopropylaminopropyl)ether.
 10. The process of claim 1 wherein the solution additionally includes an additive selected from the group consisting of antifoaming agents, antioxidants, and corrosion inhibitors.
 11. A process for the selective absorption of H₂ S from a normally gaseous mixture containing H₂ S and CO₂ comprising:(a) contacting said normally gaseous mixture with an absorbent solution comprising the amino compound of 1,2-bis-(tertiarybutylaminoethoxy)ethane, at a temperature of from 20° to 100° C. and a pressure of from 5 to 2000 psia, whereby H₂ S is selectively absorbed from said mixture; (b) heating said absorbent solution containing H₂ S at a temperature of from 50° to 170° C. at a pressureof from 1 to 50 psia, to at least partially regenerate said solution; and (c) recycling the regenerated solution for the selective absorption of H₂ S by contacting as in step (a).
 12. The process of claim 11 wherein the absorbent solution further comprises water, a physical absorbent, or mixtures thereof.
 13. The process of claim 12 wherein the absorbent solution is an aqueous solution with a concentration of amino compound in the range from about 0.1 to about 6 moles per liter.
 14. The process of claim 11 wherein the solution additionally includes an additive selected from the group consisting of antifoaming agents, antioxidants, and corrosion inhibitors.
 15. A scrubbing solution for selective absorption of H₂ S from a normally gaseous mixture containing H₂ S and CO₂ comprising:(a) an amino compound comprising a bis-secondary aminoether wherein each amino group has a severely sterically hindered secondary amino moiety; and (b) a solvent which solubilizes the amino compound and is a physical absorbent.
 16. The solution of claim 15 wherein the amino compound is represented by the general formula: ##STR5## wherein R₁ and R₈ are each independently selected from the group consisting of alkyl having 1 to 8 carbon atoms and hydroxyalkyl radicals having 2 to 8 carbon atoms, R₂, R₃, R₄, R₅, R₆ and R₇ are each independently selected from the group consisting of hydrogen, C₁ -C₄ alkyl and hydroxyalkyl radicals, with the proviso that R₂, R₃, R₆ and R₇ bonded to the carbon atoms directly bonded to the nitrogen atoms are C₁ -C₄ alkyl or hydroxyalkyl radicals when R₁ and R₈ are primary alkyl or primary hydroxyalkyl radicals, and when R₁ and R₈ are secondary alkyl or secondary hydroxyalkyl radicals, at least one of R₂ or R₃ and R₆ or R₇ bonded to the carbon atoms directly bonded to the nitrogen atoms are C₁ -C₄ alkyl or hydroxyalkyl and when R₂, R₃, R₆ and R₇ are hydrogen, R₁ and R₈ are tertiary alkyl or tertiary hydroxyalkyl radicals having 4 to 8 carbon atoms, m, n and p are positive integers ranging from 2 to 4 and o is either zero or a positive integer ranging from 1 to
 10. 17. The solution of claim 15 which additionally contains water.
 18. The solution of claims 15, 16 or 17 wherein the concentration of the amino compound ranges from about 0.1 to 6 moles per liter of total solution.
 19. The solution of claims 15, 16 or 17 wherein the concentration of the amino compound ranges from 1 to 4 moles per liter of total solution.
 20. The solution of claim 17 wherein the concentration of the solvent which is a physical absorbent ranges from about 0.1 to 5 moles per liter of total solution.
 21. A process for the selective absorption of H₂ S from a normally gaseous mixture containing H₂ S and CO₂ comprising contacting said normally gaseous mixture with an absorbent amino-compound-containing solution comprising a mixture of a di-secondary aminoether wherein each amino group is severely sterically hindered and any other amino compound, under conditions whereby H₂ S is selectively absorbed from said mixture.
 22. The process of claim 21 wherein said other amino compound is methyldiethanolamine.
 23. A process for removing H₂ S from normally gaseous streams containing same comprising contacting said gaseous stream with an absorbent solution comprising a di-secondary aminoether wherein each amino group is severely sterically hindered. 